The biceps femoris is the most injury-prone muscle in the hamstring group, accounting for roughly 84% of all hamstring strains in athletic populations. Strengthening it requires more than generic leg curls. Because this muscle has two distinct heads with different nerve supplies and different jobs, you need a mix of knee-flexion and hip-extension exercises to build it effectively.
Why the Biceps Femoris Needs Targeted Work
Your biceps femoris sits on the outer side of the back of your thigh and has two parts. The long head originates from the sit bone and crosses both the hip and the knee, meaning it extends your hip and flexes your knee. The short head attaches along the back of your thighbone and only crosses the knee, so it only bends the knee. These two heads are controlled by entirely different nerves: the long head by the tibial nerve, the short head by the common peroneal nerve. This separate wiring means your body can recruit them somewhat independently, and it means a single exercise won’t fully develop both.
MRI-based research shows that the biceps femoris long head is more active during knee flexion than during hip extension. That might surprise you if you’ve been relying on Romanian deadlifts alone to train your hamstrings. While hip-dominant movements do work the long head, knee-flexion exercises performed with the hip in a flexed position produce significantly greater muscle activation in this area.
The Highest-Activation Exercises
Electromyography (EMG) studies, which measure how hard a muscle is working during an exercise, consistently rank the Nordic hamstring curl at the top for biceps femoris activation. In one systematic review, the Nordic curl with ankle dorsiflexion produced 128% of maximal voluntary contraction in the long head. That’s more activation than the muscle generates during a max-effort isometric test, which happens because the eccentric (lowering) phase pushes the muscle beyond what it can hold statically.
Barbell deadlifts also exceeded 100% of maximal voluntary contraction in some studies, and the slip leg exercise (a sliding lunge variation) reached 99%. These three movements form a strong foundation for biceps femoris development.
Interestingly, the prone leg curl on a machine outperformed several commonly recommended exercises for overall biceps femoris activation. Romanian deadlifts, seated leg curls, glute-ham raises, and stability ball curls all produced significantly lower activation than the prone leg curl in a head-to-head comparison. This doesn’t mean those exercises are useless, but it does suggest the prone curl deserves a spot in your program if biceps femoris strength is a priority.
Knee Flexion vs. Hip Extension
The long head of the biceps femoris works during both knee flexion and hip extension, but research using MRI to measure post-exercise muscle activity found that knee flexion exercises activated the biceps femoris long head significantly more than hip extension exercises. The effect was even stronger when knee flexion was performed with the hip flexed (think: a seated or bent-over position), which places the hamstrings in a lengthened state.
This has a practical takeaway. If your routine is heavy on deadlift variations and light on curling movements, you’re likely undertraining the biceps femoris relative to what it can handle. A balanced approach includes both patterns: hip-dominant work like Romanian deadlifts and hip thrusts for the long head’s role as a hip extensor, and knee-dominant work like Nordic curls and prone leg curls for its role as a knee flexor.
How Foot Position Changes the Target
A simple technique adjustment can shift more work onto the biceps femoris during any leg curl variation. The biceps femoris is part of the lateral (outer) hamstring group, and rotating your lower leg outward during knee flexion increases its activation significantly. In one study, knee flexion strength was 9.1% greater when paired with outward tibial rotation compared to inward rotation, driven almost entirely by higher lateral hamstring recruitment.
Inward rotation of the foot, on the other hand, reduced biceps femoris activation and shifted the load toward the medial (inner) hamstrings. So if you’re doing prone or seated leg curls with the goal of targeting the biceps femoris, point your toes slightly outward. The effect is meaningful enough to influence your total force output, not just a minor tweak.
Why Eccentric Training Matters
Shorter muscle fibers in the biceps femoris long head are a known risk factor for hamstring strains. Eccentric training, where you resist a load while the muscle lengthens, is the most effective way to increase fascicle length and add structural units (sarcomeres) in series along the muscle fiber.
The Nordic hamstring curl produces the largest fascicle lengthening of any commonly studied hamstring exercise. Compared to both the deadlift and the razor curl (a hip-extension variation), the Nordic curl creates more total fascicle stretch, higher peak forces, and faster lengthening velocity during the braking phase. Research directly comparing Nordic curl training to stiff-legged deadlift training found greater increases in biceps femoris fascicle length with the Nordic curl.
That said, deadlifts and razor curls also promote fascicle lengthening to some degree, particularly because they require force production at longer muscle lengths. Training at long muscle lengths or through a large range of motion can increase fascicle length even without a strong eccentric component. So while the Nordic curl is the most potent single exercise for this adaptation, a varied program that includes hip-dominant work at full range still contributes.
Building a Biceps Femoris Program
A well-rounded approach combines three categories of movement:
- Eccentric knee flexion: Nordic hamstring curls are the priority. If you can’t perform a full Nordic, start with a partial range or use a band for assistance. Progress by increasing the range you can control on the way down.
- Loaded knee flexion: Prone leg curls with toes pointed slightly outward. Seated leg curls work too, but prone variations showed higher biceps femoris activation in comparative research.
- Hip-dominant movements: Romanian deadlifts, hip thrusts, and 45-degree hip extensions. These develop the long head’s hip extension function and build general posterior chain strength.
Training the hamstrings twice per week is a common frequency in both athletic and general strength programs. You might place Nordic curls and prone leg curls on one day, and Romanian deadlifts with a lighter curl variation on the other. Volume can range from 3 to 4 sets per exercise, with 6 to 12 reps for most movements. Nordic curls often work best in lower rep ranges (4 to 6 reps) because the eccentric demand is high.
Rebuilding After a Strain
If you’re strengthening the biceps femoris after an injury, progression follows a pain-guided approach. Exercises performed with pain at or below 4 out of 10 on a simple rating scale are considered safe and may actually speed recovery by exposing the muscle to beneficial loading earlier.
Early rehabilitation typically starts with submaximal bilateral movements: light eccentric sliders, bodyweight hip thrusts, and gentle hip-hinge patterns like the Askling diver exercise. Once you can perform bilateral eccentric sliders through full range of motion without significant pain, you progress to single-leg variations and eventually to the Nordic curl. Hip extension work follows a similar ladder, moving from bodyweight hip thrusts to loaded, single-leg, and eventually explosive versions.
Running can begin once walking is comfortable (pain 4/10 or less), starting at about 25% of your top speed and building to 50%. The jump from 80% to 100% sprint speed is where the eccentric demand on the hamstrings spikes dramatically, so that final progression happens in small increments of about 5% at a time. Return-to-sport readiness is typically gauged by pain resolution, less than 10 to 15% side-to-side asymmetry in range of motion and strength, completion of sport-specific drills, and psychological confidence in the muscle.